First draft of planex-cache command

This command is a caching wrapper for mock - replace 'mock' with 'planex-cache' to use it.

E.g. in a Makefile:
@mock --configdir=mock --quiet -r xenserver --resultdir=$(dir $@) --uniqueext=$(notdir $@) --rebuild $<
becomes
planex-cache --debug --configdir=mock --quiet -r xenserver --resultdir=$(dir $@) --uniqueext=$(notdir $@) --rebuild $<